So, I got some 'plans' for making speaker adapters, and cut them out of some old melamine shelving I had hanging around. They came out OK; they aren't going to be seen, so looks weren't that important.
I dug up my 'spare' pair of MB Quart 6.5 inchers, and set to the task of drilling the rivets out of the stock speakers. It actually went a lot better than I had expected.
I made a cheesy 'harness' to connect the Quarts to the stock wiring, mounted the first Quart and went to screw it in.
That's when I noticed the speaker surround was separating from the frame! I knew the older Quarts were notorious for needing to be reapaired, but didn't know why. Now I knkow.
So, I got out my 'Velcro' glue (remember that? ;) and glued the rubber back to the frame and 'clamped' it with a dozen clothespins. A quick inspection of the second speaker revealed even worse separation!
While waiting for the glue to dry, I drilled out the second speaker and mounted the adapter and made up the wiring. By this time it was a little more than an hour, so I checked and the glue was sufficiently dry to mount the speakers.
So, now I have 5" Quarts in the rear, and 6.5" Quarts in the front, and what a difference! It sounds pretty good. I have to turn it up a bit more than with the 'cheap' speakers that came with the car, but the quality is certainly rewarding!
If I decide to upgrade the head unit and add an amp, that's going to have to wait for Spring. It sounds decent enough as is, though!